# Homework Help: Work done during collision?

1. Aug 1, 2015

### naianator

1. The problem statement, all variables and given/known data
Two blocks have masses m1 and m2 and speeds v1 and v2. The objects slide directly toward each other along a frictionless horizontal surface and collide. After the collision, block m1 is at rest. Find the work done by the contact force on block m2 during the collision.

2. Relevant equations
W = 1/2*m*v_f^2 - 1/2*m*v_i^2

m*v_1 + m*v_2 = m*v_1' + m*v_2'

3. The attempt at a solution
m_1*v_1+m_2*v_2 = 0+m_2v_2'

v_2' = (m_1*v_1+m_2*v_2)/m_2

W = 1/2*m_2*((m_1*v_1+m_2*v_2)/m_2)^2 - 1/2*m_2*v_2^2

What am I doing wrong?

2. Aug 1, 2015

### Qwertywerty

Post edited .

Last edited: Aug 2, 2015
3. Aug 2, 2015

### Let'sthink

m_1*v_1+m_2*v_2 = 0+m_2v_2' is wrong all v-s are speeds to be corrected as
m_1*v_1- m_2*v_2 = 0+m_2v_2' ----------------------------- (1), hence
v_2' = (m_1*v_1-m_2*v_2)/m_2 or
W(by m1 on m2) = 1/2*m_2*((m_1*v_1-m_2*v_2)/m_2)^2 - 1/2*m_2*v_2^2, simplify to get the correct answer.

Work done on mass m1 by mass m2 = -1/2*m_1*v_1^2

4. Aug 2, 2015

### naianator

ahhh yes, thank you!